Editing IBM PS/2 Model 50 (section)

== Architecture ==

The Model 50 planar (Type 1, original 8550-021) and the Model 50 Z planar (Type 2, 8550-031 / 061) are distinct designs but share the chassis, the integrated VGA, the MCA slots and most I/O. Differences are concentrated around the memory subsystem.

=== Common architecture ===

* '''CPU''': Intel 80286 in a 68-pin PGA socket, 10 MHz.
* '''Math coprocessor''': Intel 80287 socket adjacent to the 80286. Optional, 8 MHz operation.
* '''MCA bus''': '''Four 16-bit MCA''' expansion slots on the planar. Each slot has both the 16-bit MCA basic edge and is keyed to accept 16-bit and 8-bit MCA cards. No 32-bit MCA slots; no ISA compatibility.
* '''Integrated VGA''' on the planar with the IBM VGA gate-array implementation and an Inmos IMSG171P-style RAMDAC. Modes: 80×25 / 80×50 text, 640×480×16, 320×200×256, 720×400×16 text (8×16 character box). Output on 15-pin DSUB at rear. Analog monitors only.
* '''Integrated I/O''':
** PS/2 keyboard (Mini-DIN-6).
** PS/2 pointing-device port (Mini-DIN-6) — the keyboard and mouse ports are functionally identical at the 8042 controller but the BIOS expects each device on its designated port.
** Asynchronous serial (DB-25 male).
** Centronics parallel (DB-25 female).
* '''Floppy controller''': PS/2 single-cable interface integrated on the planar, 1.44 MB 3.5" half-height drive.
* '''Hard drive interface''': '''None on the planar''' — the hard drive is supplied through an MCA controller card. The drive itself plugs directly into a connector on the rear edge of the controller card, eliminating a separate ribbon cable to the planar. The Model 50 (8550-021) uses the '''IBM Fixed Disk Adapter/A''' (ST-506); the Model 50 Z 30 MB (8550-031) uses the same ST-506 controller; the Model 50 Z 60 MB (8550-061) uses the '''IBM ESDI Fixed Disk Adapter/A'''.
* '''BIOS''': 128 KB total, split between '''CBIOS''' (Compatible BIOS, for backward DOS compatibility) and '''ABIOS''' (Advanced BIOS, for protected-mode OS/2). Held in two 64 KB EPROM / mask-ROM chips at U-positions on the planar.
* '''CMOS / RTC''': Battery-backed via the '''Dallas DS1287''' / DS12887 integrated RTC + lithium-cell module. Stores SETUP configuration, date / time.
* '''Audio''': PC speaker only.

=== Memory subsystem ===

* '''Model 50 (8550-021)''': '''Two''' 30-pin PS/2 SIMM sockets. Accept only '''512 KB''' parity SIMMs. 1 MB on planar (2 × 512 KB). Maximum planar memory: 1 MB. Additional RAM via MCA memory cards up to a system maximum of 7 MB.
* '''Model 50 Z (8550-031 / 061)''': '''Single''' 30-pin PS/2 SIMM socket. Accepts '''1 MB''' or '''2 MB''' parity SIMMs. Maximum planar memory: 2 MB. Additional RAM via MCA memory cards up to a system maximum of 7 MB.

PS/2 SIMMs use an IBM-specific pinout that is similar to but '''not''' interchangeable with standard PC/AT 30-pin parity SIMMs. The originals on the Model 50 carried silver-cap "metal-can" DRAM chips; any PS/2-pinout SIMM with the right capacitance and speed (125-ns on Model 50, 85-ns on Model 50 Z) works as a replacement. The AST Advantage/2 16-bit MCA card with 4 MB pre-installed plus a SCSI controller is a documented aftermarket option that takes the Model 50 to its 7 MB ceiling.

=== Storage ===

* '''Floppy''': 1.44 MB 3.5" half-height, PS/2 single-cable interface. Drives are typically ALPS, Mitsubishi, Sony or YE-Data.
* '''Hard drive''' (8550-021): Seagate ST-506 20 MB (the famously slow 85 ms-access-time drive PC Magazine reviewers criticised) on the IBM Fixed Disk Adapter/A MCA card.
* '''Hard drive''' (8550-031): Seagate ST-506 30 MB on the IBM Fixed Disk Adapter/A. Twice the seek-speed of the 20 MB drive.
* '''Hard drive''' (8550-061): 60 MB ESDI on the IBM ESDI Fixed Disk Adapter/A.

The ESDI Fixed Disk Adapter/A is the same MCA controller used in the Model 60 (70 MB), Model 80 (70 MB, 115 MB), and other ESDI-equipped PS/2 systems. The drive plugs directly into a rear-edge connector on the controller card.

=== Chassis ===

The Model 50 chassis is a low-profile desktop, 360 W × 420 D × 140 H mm, designed to sit under a monitor on a desk. Two thumbscrews at the rear release the cover, which slides back about 10 mm and lifts off. Inside, the planar mounts on the floor of the chassis; the MCA riser is at the rear; the drive cage runs across the front-right with the floppy and the hard drive controller card directly behind it. The PSU is on the left-front.

Drives are held by IBM's plastic captive push-pin tabs (the system shipped with a small plastic "crowbar" tool to release them). MCA cards are released by plastic thumbscrews on the rear bracket.

=== Power Supply ===

'''94 W''' switching power supply, 110/220 VAC auto-sensing. PS/2 single-cable drive power feed integrated on the planar — '''no Molex 4-pin connectors'''. The 94 W is borderline for a fully-populated Model 50 with multiple MCA cards plus an ESDI drive; the Model 50 Z PSU is electrically the same.